1. Field of the Invention
A liquid crystal display containing a ferroelectric liquid crystal material, and a fabrication method thereof.
2. Description of the Related Art
Liquid crystal displays are one type of flat panel displays that have attracted continuous attention in recent years. Liquid crystal displays operate by varying the optical anisotropy of liquid crystals by applying an electric field to a liquid crystalline material that combines the fluidity of liquid and the optical property of crystal. Liquid crystal displays currently find wide use due to their low power consumption, small volume, large size and high definition, when compared with a conventional cathode ray tube.
A liquid crystal display can be set in various alignment modes using the properties and the alignment pattern of the liquid crystal. A typical liquid crystal display is a twisted nematic (TN) liquid crystal display.
In a TN liquid crystal display, a liquid crystal director is arranged and twisted at 90.degree. between upper/lower substrates, and this director is controlled by applied voltage. When no electric field is applied, the TN liquid crystal display has a maximum transmittance in upper and lower directions, and has a symmetric direction of the viewing angle. When an electric field is applied, liquid crystal molecules gradually align in parallel with the direction of the electric field if the liquid crystal molecules have a positive dielectric anisotropy. When greater voltages are applied between the substrates, the liquid crystal molecules rearrange in the direction of the electric field, thereby having a polar angle of 90.degree. and being aligned normal with and between the upper and lower substrates. Accordingly, the TN liquid crystal display has a drawback in that it cannot realize a wide viewing angle due to a contrast ratio (C/R) and a luminance that greatly varies with the viewing angle.
Several ways have been proposed to overcome the drawbacks of inadequate viewing angle resulting from a vertical electric field. Among of them, an in-plane switching liquid crystal display is typical.
FIG. 1 shows a sectional view illustrating the variation of a molecular arrangement of a liquid crystal with ON/OFF voltages in a related art liquid crystal display.
As shown in FIG. 1, when no voltage is applied (OFF) to a pixel electrode 21 and a common electrode 22, liquid crystal molecules 5 align in parallel with the pixel electrode 21 and the common electrode 22.
When voltage is applied (ON) to the pixel electrode 21 and the common electrode 22, an in-plane electric field 60 forms between the pixel electrode 21 and the common electrode 22, and the liquid crystal molecules 5 horizontally align along the in-plane electric field 60.
However, the related art in-plane switching liquid crystal display has disadvantages arising from the aperture ratio and the transmittance of light radiated from a bottom light source deteriorating because the liquid crystal display panel's lower substrate 1 bears all of the pixel electrode 21, the common electrode 22 and their electrode wires (not shown) necessary generate the in-plane electric field 60. In contrast, the upper substrate 2 has no obstructions of this type. Therefore, obtaining sufficient luminance becomes difficult in the related art in-plane switching liquid crystal panel. In contrast, the upper substrate 2 is unobstructed.
FIG. 2 shows a partially enlarged sectional view of a related art liquid crystal display having a ferroelectric liquid crystal coated on an alignment layer of an underlying lower substrate.
As shown in FIG. 2, an alignment layer 111 is coated on the lower substrate 101, and a ferroelectric liquid crystal 141 is coated on the alignment layer 111. After coating, the ferroelectric liquid crystal 141 undergoes a phase transition to form a ferroelectric liquid crystal layer 141.
The ferroelectric liquid crystal 141 expresses a spontaneous polarization during its phase transition. The expressed spontaneous polarization has a strong polarity due to its high electron density. Therefore, the ferroelectric liquid crystal 141 is coated on the alignment layer 111 of the underlying lower substrate 101.
In the liquid crystal display having the coated ferroelectric liquid crystal 141, the ferroelectric liquid crystal undergoes a phase transition while exposed to air. However, the ferroelectric liquid crystal 141 is not exposed to air at the alignment layer 111.
When the liquid crystal expresses its spontaneous polarization, the liquid crystal has a rotation direction toward the lower substrate 101 due to the strong polarity of the expressed spontaneous polarization, the non-polarity of the air, and the relative polarity of the alignment layer.
FIG. 3 illustrates a schematic enlarged sectional view of a portion of a related art liquid crystal display using a ferroelectric liquid crystal. As shown in FIG. 3, lower and upper substrates 101 and 102 respectively have ferroelectric liquid crystal layers 141 and 142 of FIG. 2. When a bulk nematic liquid crystal 200 is injected between the attached lower and upper substrates 101 and 102 having alignment layers 111 and 112, ferroelectric liquid crystals 141a and 142a stabilize their rotations respective to each other due to their reverse rotation directions.
FIG. 4 shows defects of the related art liquid crystal display containing a ferroelectric liquid crystal.
FIG. 4 shows a related art ferroelectric liquid crystal display that includes a lower substrate 101 having a first alignment layer 111 and a first ferroelectric liquid crystal layer 141 that has passed though a phase transition. The upper substrate 102 attaches to the lower substrate 101 and has a second alignment layer 112 and a second ferroelectric liquid crystal layer 142 that has also passed through a phase transition. A bulk nematic liquid crystal 200 is injected between the attached lower and upper substrates 101 and 102.
As shown in FIG. 4, the related art liquid crystal display includes an effective phase delay layer 201 and a rotation restraint layer 203 that are respectively influenced by the alignments of the first and second ferroelectric liquid crystal layers 141 and 142 when a voltage is applied.
The effective phase delay layer 201 and the rotation restraint layer 203 tend to rotate in opposite directions. That is, the effective phase delay layer 201 and the rotation restraint layer 203 have reverse rotations.
In other words, a reverse rotation force of the rotation restraint layer 203 is applied against a forward rotation force of the effective phase delay layer 201, and the liquid crystal 200 does not rotate in the direction of the electric field to thus make its rotation state unstable.
FIG. 5 is a graph illustrating the disadvantageous and defective transmittance and a response speed in the related art liquid crystal display of FIG. 4.
As shown in FIG. 5, the X-axis denotes time in milliseconds (ms), and the Y-axis denotes a relative transmittance in percentage (%). A solid line denotes the transmittance when a voltage is applied (ON), and a dotted line denotes the transmittance when no voltage is applied (OFF).
When no voltage is applied (OFF) to the liquid crystal display, the liquid crystal display cannot be distinguished from other liquid crystal displays in FIG. 5. However, when voltage is applied (ON) to the liquid crystal display, the transmittance versus time shows oscillating switching.
In other words, the related art liquid crystal display has drawbacks arising from the liquid crystal being primarily, i.e., forward rotated and then reverse rotated by a fixed force. The interaction of the two rotation forces serves to deteriorate the luminance. Also, when the liquid crystal again rotates in response to another switching signal, this rotation prevents an effective phase delay and causes a slow response speed. As a result, the related art ferroelectric liquid crystal displays show distinct disadvantages that limit their effectiveness in modern display applications.